The recruitment of motor units in a muscle is generally understood to progress in a fixed order from units that exert small forces to units that exert large forces. However, this understanding has been questioned in the literature as studies have shown that motor units in some muscles are recruited differently for different directions of joint force. However, the physiological basis for the differential activation of motor units has not been established. Using the first dorsal interosseous (FDI) as a model multifunctional muscle, we will determine if different motor units within the FDI exert twitch forces with different ratios of abduction to flexion force. If different motor units have a greater mechanical advantage for one direction over another, we will determine if the central nervous system (CMS) activates motor units so that units with the greatest mechanical advantage for the given task are preferentially activated. Loss of directional control in the affected hand after stroke may relate to a breakdown in the selective activation of motor units rendering some directions of index finger force either inaccessible or uncontrollable. Our study will determine the feasibility of this hypothesis by seeking to establish the physiological basis of differential motor unit activation in unimpaired human subjects. We will assess motor unit twitch force direction using spike- triggered averaging (STA). STA may accurately determine motor unit twitch force direction despite the fact that it can not accurately estimate the absolute amplitude of a twitch. We will examine the consistency of STA estimates for twitch direction across different directional tasks. By studying many motor units, we will assess the distribution of twitch directions that can be generated by FDI motor units. We will determine if a correlation exists between the twitch direction of a motor unit and its recruitment. Our study will both determine whether STA is a reliable indicator of motor unit twitch force direction, and determine whether different twitch force directions among motor units forms the physiological basis for differential motor unit activation. Relevance to Public Health: After suffering a stroke, many individuals lose the ability to move their joints in particular directions. Our study seeks both to develop methods to examine the direction of movement controlled by single cells in the spinal cord, and to determine how the undamaged brain activates these cells to control different directions of joint movement. Having this information will give us a baseline which we can then use to better understand how the damaged brain controls the spinal cord, and how specific interventions may improve motor control post-stroke. [unreadable] [unreadable] [unreadable]